Nitrogen-phosphine laser (u)

ABSTRACT

A gas laser utilizes vibrationally excited nitrogen as an energizing gas and, through resonant collisions with a lasing gas, transfers the energy to the lasing gas, preferentially to an upper laser energy level. The lasing gas is phosphine, which is preferably introduced directly into the laser cavity free of prior excitation so as to avoid molecular dissociation. Laser energy at 8.3, 7.5 and 30.0 microns is produced.

United States Patent [72] Inventors Barry R. Broniin Weathersfield;

Daniel J. Seery, Glastonbury, Conn. [21] Appl. No. 786,484 [22] FiledDec. 23, 1968 [45] Patented Apr. 6, 1971 r [73] Assignee 7 UnitedAircraft Corporation East Hartford, Conn.

[54] NlTROGEN-PHOSPHINE LASER (U) Transfer and Optical Maser Action in N2- C02.

. Physical Review Letters Vol. 13. No. 2] Nov. 1964 pp 617- 6 l 9.Primary Examiner--Rodney D. Bennett, Jr.

Assistant Examiner-Daniel C. Kaufman Attorney-Melvin Pearson WilliamsABSTRACT: A gas laser utilizes vibrationally excited nitrogen 40mins 1Drawing as an energizing gas and, through resonant collisions with a[52] [1,5, Cl 331/945, lasing gas, transfers the energy to the lasinggas, preferentially 330/43 to an upper laser energy level. The lasinggas is phosphine, [51] Int. Cl r101; 3/22 which is preferably introduceddirectly into the laser cavity [50] Field of Search 331/945; free ofprior excitation so as to avoid molecular dissociation.

330/43 (Abstracts) Laser energy at 8.3, 7.5 and 30.0 microns isproduced.

r JdOPCf Q4 w A W 2- 6/7.: v 'Xd/TflT/dd/ ,4 v/7 A/z v M as-.. swan.

NI'I'ROGEN-PIIOSPI'IINE LASER (U) BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates to gas lasers, and moreparticularly to a gas laser for producing radiation in the infrared.

2. Description of the Prior Art The mechanics of gas lasers arecurrently well-known. A great deal of attention has been paid recentlyto the excitation of a lasing gas to an energy level capable of emittingphotons and thereby participating in stimulated emission of coherentlight within'a laser cavity as a result of energy preferentiallytransferred to the lasing gas through'near-resonant collision with avibrationally excited energizing gas. One type of gas laser which hasreceived a great deal of attention is the nitrogen-carbon dioxide laser.In this type of gas laser, the nitrogen is excited to the firstvibrational level, and it transfers energy into the upper laser level(001) of carbon dioxide preferentially, so as provide a populationinversion which supports laser emission. The emission of photons by thecarbon dioxide causes the molecules thereof to assume the enery of thelower laser level (100) and molecules in this energy level rapidly decayvia gas collisions to the ground state.

The method of exciting the energizing gas may vary in accordance withthe particular design parameters of a given laser, as is known in theart. For instance, electric excitation may be utilized in any of severalforms. One well-known form is radiofrequency excitation; another fonn isdirect current plasma excitation; and a third known form is microwaveexcitation. Additionally, it is possible to excite the energizing gas bycausing it to absorb intense light of a frequency matched to the spacingof the low-lying vibrational energy levels of the energizing gas. Forinstance nitrogen may be raised to its first vibrational level byabsorption of light with a wavelength of about 4.3 microns, and carbonmonoxide may be raised to its first vibrational energy level byabsorption of light with a wavelength of about 4.7 microns. Anotherknown method of obtaining energizing gas in an excited state comprisesthe sudden cooling of a heated energizing gas. Thus, the energizing gasmay be heated as a result of the utilization of any thermal source (suchas the simple burning of fuel) or be a heating are, and it maythereafter be caused to flow through an expansion nozzle at supersonicspeeds so as to freeze the energy in the lower vibrational levels whiletranslational cooling takes place, thusproviding a highly nonequilibriumpopulation distribution with preferential excitation at these levels. inthe preferred embodiment of the invention, gas temperatures equal to1,000 K. or greater will be achieved in the energizing gas beforeexpansion.

A recent advancement in the art comprises the technique of mixing,wherein the lasing gas is caused to mix intimately with thevibrationally excited energizing gas directly within the laser cavity toaccomplish population inversion in the lasing gas so that the energizingof the lasing gas to its upper laser level or levels does not causeprelasing, and is not depleted by collisional energy transfers or otheradiabatic phenomena prior to entrance into the laser cavity where theenergy may participate in the generation of laser light.

In the aforementioned gas lasers, the wavelength of the laser radiationobtained depends upon the laser transitions between various energylevels. A great deal of work has been done with the carbon dioxide laserwhich produces laser light at l0.6 microns, and with other laser systemswhich produce light at various other wavelengths. These are well suitedto some uses, but atmospheric absorption of light at certain frequencieslimits their usefulness. Additionally, laser energy may be utilized foruseful processes, in whichit becomes important to stimulate chemicalreactions and other molecular transitions, provided that light of aproper wavelength can be obtained (a well-known process calledphotolysis).

It should be understood that the laser emission occurs over a band ofwavelengths approximately centered on the wavelengths listed.Furthermore, due to uncertainties existing in the present art, onlyapproximate assignments of these laser wavelengths can be made.

SUMMARY OF INVENTION The object of the present invention is to provide alaser capable of radiation at infrared wavelen According to the presentinvention, nitrogen is utilized as an energizing gas to vibrationallyexcite phosphine to an upper laser level in a gas laser, wherebystimulated coherent emission of electromagnetic radiation or laserenergy will result at an infrared wavelength.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of a preferred embodiment thereof, as illustratedin the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE herein comprises aschematic block diagram of a laser system in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the FIGURE, asource 1 of molecular nitrogen energizing gas delivers the gas to avibrational excitation means 2. The vibrational excitation means 2 maycomprise an electronic discharge of either the direct current, radiofrequency, or microwave variety, as is known in the art. It may on theother hand comprise a source of intense light of approximately 4.3microns.,lt may also comprise means for thermally heatingthe energizinggas and for thereafter suddenly cooling it, such as by passing itthrough a rapid expansion nozzle to freeze the vibrational energystates. All of these techniques are well-known in the art, and thechoice of one of them is not germane to the present invention. Thevibrationally excited energizing gas passes from the vibrationalexcitation means 2 into a laser cavity 3 which also receives phosphinefrom a source 4 of lasing gas. The lasing gas is preferably introducedinto the laser cavity at a relatively low temperature, such as roomtemperature. Methods of accomplishing efiicient mixing of gases areknown in the art. This causes intimate mixing of the vibrationallyexcited energizing gas with the lasing gas within the optical cavity sothat essentially each molecule of lasing gas which is brought to anupper laser level of energy will emit photons directly within the lasercavity, therefore avoiding prelasing or spontaneous vibrationaladiabatic equilibration processes which would remove molecules of lasinggas from the upper laser level of excitation by other than the emissionof a photon. Additionally, the use of the mixing configurationillustrated in the FIGURE is preferred because phosphine is a relativelyfragile molecule, since the dissociation of the P-H bond is 76.4kcallgm/mol. Hence it would in general be deleterious to have phosphineaccompany the nitrogen in the excitation stage where chemicaldecomposition of the phosphine might ensue.

The flow of the gas from the source 1, through the vibrationalexcitation means 2 and the laser cavity 3 is caused by a suitable pumpand effluent control means 5. The pump and effluent control means 5 mayprovide not only for flow through the system, but supersonic flow in thecase where an expansion nozzle is utilized, or high flow at relativelylow pressure in the case where an electric discharge is involved in thevibrational excitation means 2. The pump and effluent control means 5may also provide for the combustion of the effluent so as to avoidreleasing toxic phosphine. As an alternative, however, a leaktightclosed cycle system may be employed if desired, without altering thepractice of the essential teachings of the present invention.

The technique of mixing the preferably room-temperature lasing gas intothe excited energizing gas promotes a favorable Upper Lower EA:Approximate level level cm. wavelength, A: 0010 0001 1, 206 8. 3 00100100 1,336 7. 5 0010 0200 334 30.

The desirable depletion of lower laser levels can be enhanced throughthe addition of other gases which are efficient agents for transferringvibrational energy to translational energy, such as He, H,, H,0.However, the PH molecule itself exhibits high vibrational energytransfer efficiency, so that the need for relaxing additives is minimal.

To produce self-sustained lasing in the flow, a highlypolished mirrorcavity should be provided to view" the mixing region of the flow. Thiscavity should be coextensive with the region of flow containing a finiteexcited state population inversion density.

From the foregoing laser transitions, it can be seen that the presentinvention will provide laser radiation at TWO wavelengths which are notsignificantly absorbed in the atmosphere. These are in the vicinity of8.3 and 7.5 microns. Additionally, the present invention provides lightof a wavelength which is well suited to particular uses not requiringextensive transmission through the atmosphere.

Although the invention has been shown and desired with respect topreferred embodiments thereof, it should be understood by those skilledin the art that various changes and omissions in the form and detailthereof may be made therein, without departing from the spirit and thescope of the invention.

We claim:

l. The method of operating a gas laser having a laser cavity comprising:

flowing an energizing gas through said cavity, said energizing gasconsisting of vibrationally excited molecular nitrogen; and

contacting a second gas stream with said energizing gas in t the lasercavity, the second gas stream consisting of a lasing gas which ispredominantly phosphine, whereby the energy of said energizing gas istransferred to the lasing level of the lasing gas.

2. 1n the method according to claim 1, the further step which comprises;adjusting the optical parameters of said laser cavity so as to providelaser radiation over a band of wavelengths approximately centered at30.0 microns.

3. In the method according to claim 1, the further step I whichcomprises; adjusting the optical parameters of said laser cavity so asto provide laser radiation over a band of wavelengths approximatelycentered at 8.3 microns.

4. In the method according to claim 1, the further step which comprises;adjusting the optical parameters of said laser cavity so as to providelaser radiation over a band of wavelengths approximately centered at 7.5microns.

2. In the method according to claim 1, the further step which comprises; adjusting the optical parameters of said laser caVity so as to provide laser radiation over a band of wavelengths approximately centered at 30.0 microns.
 3. In the method according to claim 1, the further step which comprises; adjusting the optical parameters of said laser cavity so as to provide laser radiation over a band of wavelengths approximately centered at 8.3 microns.
 4. In the method according to claim 1, the further step which comprises; adjusting the optical parameters of said laser cavity so as to provide laser radiation over a band of wavelengths approximately centered at 7.5 microns. 